This invention relates to a thermo-pneumatic pump, particularly for indicating the state of fill in liquid containers by means of an immersion tube. The pump includes a pump chamber having at least one valve, a heating conductor arranged in the pump chamber, and an electric switch controlled by the heating conductor.
In indicating a state of fill by means of an immersed tube, a small thin tube is arranged vertically in the liquid to be monitored and is immersed to an extent corresponding to the measuring range. A stream of gas is introduced into the immersed tube and displaces the liquid in the tube. The hydrostatic pressure that builds up when this happens is a measure of the depth to which the tube is immersed and for the state of fill of the container to be monitored. For reasons of safety, use is made of a small excess of gas so that a little gas continuously emerges from the end of the immersed tube. Apart from hand pumps comprising a piston or a diaphragm, what are known as thermo-pneumatic pumps are used for generating the gas pressure.
German Pat. Nos. OS 18 05 479 and 19 15 743 disclose thermo-pneumatic pumps comprising heating conductors coiled through several turns, in which periodic heating and cooling of the heating conductor which, in conjunction with suitable valves, finally imparts movement to the gas, are controlled by an external electronic impulsing means. Although the desired object of rendering the switching operations independent of temperature is achieved, the expense involved is considerable because of the electrical components, such as transistors, timing devices, etc., that are required. Since the subject matter of the invention is an article that is to be mass-produced at a reasonable cost, the known solution was abandoned. The main purpose to which such thermo-pneumatic pumps are put is that of indicating the state of fill in the fuel tanks of motor vehicles. Recently, checking of the oil level in the crank chambers of internal combustion engines has also assumed greater importance. In this connection, a particularly accurate indication over an extremely narrow measuring range of a few millimeters water column is required. This need imposes particularly stringent requirements as regards the precision and reliability not only of the pressure-measuring instrument used, but also of the thermo-pneumatic pump.
German Pat. No. 12 49 550 also discloses a thermo-pneumatic pump of the kind initially described in which automatic control is achieved by means of a bimetal switch. In this pump, the heating conductor acts directly on the bimetal switch. However, a considerable delay factor is involved in the transmission of heat from the heating conductor to the bimetal. Furthermore, because of the mass of the bimetal, a bimetal switch has a considerable thermal inertia which manifests itself when the heat of the heating conductor acts directly on the bimetal. The two-fold thermal inertia makes it necessary to operate the heating conductor under low power or at low temperature so as to avoid destruction of the heating conductor if the system remains switched on too long. However, operation at low temperature leads to marked dependence upon the ambient temperature. In this connection it has to be borne in mind that the energy balance is adversely influenced by the dissipation of heat from the heating conductor itself and from the bimetal which is large in terms of area. Because of the unavoidable lengthy cycles, a pronouncedly discontinuous mode of operation results which manifests itself in a continuous oscillation affecting the indication. Such an apparatus cannot be used for monitoring the oil level in an internal combustion engine.